1. Field of the Invention
This invention relates to a double inversion buffer for use in integrated circuits and, more particularly, relates to a double inversion buffer having a short low-to-high propagation delay which is obtained by using an active pull-down means.
2. Description of the Prior Art
Buffers are employed on almost all integrated circuits. They serve to isolate the circuit from external noise and ensure that voltage levels for the digital inputs are consistent and meet the internal requirements of the circuit. In order to transfer the correct digital data from external sources to the circuitry within an integrated circuit, double inversion buffers are typically employed. Thus, the digital data is inverted once thereby creating the inverse data and is then inverted a second time to restore the signal to the original digital data at a specified, stable voltage level. Preferrably, to provide a good low voltage, a sinking current is employed to insure that charge is removed from a given node, thereby actively pulling the node low quickly. Conversely, to produce a good high voltage, current is typically provided (or "sourced") thereby to insure that ample current is provided on a given node to produce a high voltage.
Conventional double inversion buffers employ a first circuit means to produce an inversion of the digital input, a second inversion means to restore the state of the original input, and output means which includes both a pull-up means and a pull-down means to provide the requisite high or low outputs as dictated by the twice inverted representations of the digital input signal. Various prior art examples of double inversion buffers are shown in FIGS. 1-3. The operation of these examples is discussed in detail subsequently. These conventional circuits possess significant propagation delay times. For high-speed integrated circuits it is desirable to minimize the double inversion propagation delay time. The limiting factor in reducing the low-to-high propagation delay time, TPLH, is known to be the time that it takes to turn off the transistor which pulls down the output node. This is akin to regeneration which must necessarily be accomplished before the next digital input can be accepted and inverted twice.
It is therefore an object of the present invention to provide a double inversion buffer which has a low propagation delay time.
It is another object of the present invention to provide a double inversion buffer circuit which has a fast pull-down time for the output transistor.
It is an additional object of the present invention to provide a double inversion buffer which utilizes active means to pull down the output transistor.